The present invention generally relates to the adsorption of heavy metals and, more particularly, to the adsorption of heavy metals such as Cs and Sr by keratin protein such as that found in feather fibers.
In a multitude of applications and environments, there is often a need to remove contaminants from solids, liquids, and gases. Those contaminants may be in one of many forms, including heavy metal ions. As one example, the U.S. Department of Energy (DOE) has generated large volumes of high-level radioactive water waste (HLW) from the reprocessing of spent uranium. The HLW contains significant levels of radioactive strontium (Sr) and cesium (Cs). The HLW is stored in stainless steel tanks underground. The HLW has been made highly alkaline (about pH 13) so as to not attack the tank material and leach through the walls. The high alkalinity of the material causes it to range in temperature between about 25 to 93xc2x0 C. in the tanks.
The tanks can leak over time and cause radioactive environmental cleanup issues. For reasons of safety, the DOE wants to remove the HLW from the tanks and close the storage site. To date, the systems proposed to reduce the levels of Sr and Cs are expensive or capital intensive, with each having issues with them. For example, ion exchange resins have been evaluated in the environment. They, however, tend to be very expensive and to increase significantly in volume under use. A DOE report entitled xe2x80x9cElectrochemical Treatment of Alkaline Nuclear Wastesxe2x80x9d, (DOE Report DOE/EM-0560) (January 2001) describes a process that electrochemically converts aqueous sodium nitrate/nitrite into sodium hydroxide and then chemically reduces the nitrogen species to gaseous ammonia, nitrous oxide, and nitrogen. This process is complex and capital intensive.
Keratin protein derived from natural sources has been shown to have an affinity for heavy metals (Tratnyek, J. P., xe2x80x9cWaste Wool as a Scavenger for Mercury Pollution in Watersxe2x80x9d, Environmental Protection Agency, U.S. Printing Office, Washington, D.C., 1972). Keratin is a sulfur containing protein that may be found in epidermal outgrowths, such as feathers, horns, hoofs, hair, and wool. Given the naturally occurring and abundant sources of keratin, much interest has focused on its use for adsorbing metals, whether for eventual recovery or disposal.
In particular, keratin protein derived from chicken feathers has been used to adsorb gold, platinum, and palladium at about a desirable pH between 1 to 3. Suyama et al., xe2x80x9cBiosorption of Precious Metal Ions by Chicken Feathers,xe2x80x9d Applied Bio. and Biotech. Part A: Enzyme Eng. and Biotech. Spring 1996, 57-58:67-74. Similarly, keratin protein was used to remove rhodium, ruthenium, gold, silver, iridium, zinc, aluminum, iron, copper, nickel, and tin preferably at a pH between 2 to 3 in U.S. Pat. No. 4,289,531. Maximum adsorption of gold-cyanide by chicken feathers has occurred around pH 2. Ishikawa et al., Recovery and Refining of Gold by Gold Cyanide Ion Biosorption Using Animal Fibrous Proteins, Applied Bio. and Biotech. Part A: Enzyme Eng. and Biotech. Spring 1998, 70-72:719-728.
Yet, the absorption process by keratin protein has been found to be slow, somewhat limited in absorption capacity in comparison to other adsorbents, and dependent upon a low pH. It is also generally accepted that naturally occurring keratin fibers have poor resistance to alkali treatment and can lose much of their mechanical strength when subjected to a high pH environment of about pH 9 to 14.
As can be seen, there is a need for a keratin protein method of adsorbing heavy metal ions by keratin protein, such as Cs and Sr, that can be used in a high pH environment of about 9 to 14 and also in a highly concentrated alkaline environment.
In one aspect of the present invention, a method of removing a heavy metal from an alkaline composition comprises providing a keratin protein from a feather; treating the keratin protein with ultrasonic or the like; making a slurry of the treated keratin protein; contacting the keratin protein with the alkaline composition of heavy metal by mechanical agitation or the like; and filtering a supernatant produced in the step of contacting.
In another aspect of the present invention, a method of removing at least one of Sr and Cs from an alkaline solution having a pH between about 9 to 14 comprises providing a keratin protein from a poultry feather (or turkey, duck, etc.); making a slurry of the keratin protein and the alkaline solution; treating the slurry with ultrasound; contacting the keratin protein with at least one of Sr and Cs by mechanical agitation or the like; and filtering a supernatant produced in the step of contacting.
In a further aspect of the present invention, a method of removing both Sr and Cs from an alkaline solution having a pH between about 9 to 14 comprises providing a keratin protein from essentially a fiber portion separated from a quill portion of a chicken feather; making a slurry of the keratin protein and the alkaline solution and subjecting the slurry to ultrasound, the Sr being present in the alkaline solution at least at about 5 ppb and up to about 100,000 ppb or more and the Cs being present in the alkaline solution at least at about 5 ppb and up to about 100,000 ppb or more; contacting the keratin protein with the Sr and Cs by mechanical agitation or the like, the step of contacting occurring at a temperature between about 20 to 90xc2x0 C. and a pressure not greater than about 10 psi; and filtering a supernatant produced in the step of contacting.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.